1. Field Of The Invention
The present invention relates to a solid oxide fuel cell and a porous electrode to be used therefor.
2. Related Art Statement
Recently, fuel cells have been noted as power generating devices. Such a fuel cell is a device capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique owing to its high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electric power generation efficiency without being affected by the scales of installations.
Particularly, since the solid oxide fuel cell (referred to as "SOFC" hereinafter) operates at high temperatures of 1,000.degree. C. or more, activity of electrodes is very high. Thus, catalyst of a noble metal such as expensive platinum entirely unnecessary. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously much higher than those of other fuel cells. Furthermore, since constituent materials are all solid, the SOFC is stable and has long service life.
In order to produce such an SOFC, it has been proposed, for example, to form a one end-closed cylindrical porous support tube from an air electrode material having ion conductivity and electron conductivity, and successively form a solid electrolyte film and a fuel electrode film on a surface of the porous air electrode tube. An oxidizing gas is supplied into an internal space of the porous air electrode tube, while a fuel gas, such as H.sub.2, CH.sub.4 or the like is caused to flow along the outer periphery of the fuel electrode film. Consequently, the fuel gas reacts with oxygen ions, on the surface of the fuel electrode film, which have diffused through the solid electrolyte film. As a result, electric current flows between the air electrode film and the fuel electrode film to make it possible to use the SOFC as a cell for generating electric power.
In order to put the SOFC to practical use, generated power density per unit area of the cell needs to be increased for lowering its power generating cost. In order to increase the generated power density, it is required to enhance the diffusion of reactive gases in pores of the porous electrode material, to elevate the surface contact density of three-phase interface where cell reaction actually proceeds at the interface among the solid electrolyte and the electrode materials, and to lower the resistance to ion conductivity of the solid electrolyte film and electron conductivity of the electrode film.
In order to enhance the diffusion of reactive gases in pores of the porous electrode material, it is preferable to form the porous air electrode from a material containing pores which have a large diameter and are not largely curved. However, if the solid electrolyte film is formed on a surface of the material having large pores, the surface contact density of the three-phase interface could not be large. On the other hand, in order to make the surface contact density of the three-phase interface, large if a porous material containing small pores is used, the resistance to diffusion of gases in the porous air electrode becomes large. The same is also applicable to the fuel electrode film.